Accelerator Stuck Open at 4,500 rpm!
 

Excellent Don!

I just had something strange happen to my '90.
Crossing a highway under green I goosed the engine and hit the brakes on the other side of the intersection.
The accelerator stuck open -4500 RPM!
Wouldn't return to normal after a hot restart.
The linkage is free so I'm thinking TPS or ECU?
Have the bonnet open to help cool it down and disconnected the battery to reset the ECU.
Any thoughts?

Hi Roger,

I've moved your post from the thread on oil cooler lines to start your own thread, since yours is a very serious issue!

Since the throttle linkage is free, what about the throttle plate/butterfly valve? Any chance it stuck open? The TPS would be another suspect, but I don't know how it could cause the engine to run at 4,500 rpm if the butterfly valve was closed and preventing sufficient air from entering the intake. If the Idle Air Control Valve stepper motor stuck open it could lead to high idle, but again, I don't know if it could be enough for the engine to run at 4,500rpm. Any chance the accelerator pedal got caught on the floor mat?

Cheers,

Don

The floor mat is OK
I'll check the throttle plate when it cools down

Thx

Could something in the accelerator pedal pivot be loose or worn and have caught? Could your kickdown switch have stuck on (part 22 below)?

https://cimg5.ibsrv.net/gimg/www.jag...5e87d9e18b.png

The parts list is here:

https://www.jaguarclassicparts.com/u...-from-v-594576

Cheers,

Don

I checked the throttle plate and all was well re moved freely, no binding open or closed and so on
I disconnected the battery to let the ECU reset and all is well.
Took for a decent drive around 4:30 and all is well.
Could hard acceleration and fast hard braking mess with the ECU, TPS?
Gotta be everything else appears to be as it should..? Puzzling for sure.

Hi Roger,

Just trying to think this through:

1. On an XJ40, the TPS is purely a passive potentiometer and the throttle has no built-in motor that can open the valve; the throttle valve is strictly mechanical, controlled by the accelerator pedal and cable linkage;

2. If the TPS signal were corrupted such that the ECM read the throttle as wide open (WOT), it would increase the injector pulse duration to allow more fuel into the combustion chambers;

3. But it wouldn't matter how much the ECM increased fueling if the throttle valve did not open to allow sufficient air for proper combustion. If the throttle remained closed or not sufficiently open, the Air-Fuel Ratio might get so rich that it could not ignite, the definition of flooding, but the rpm would only rise a bit before the engine stumbled and choked and died.

The above points make me think that for your engine to have stuck at 4,500 rpm, either the throttle must have stuck open, or a large air leak must have opened up somewhere in the plumbing between the throttle and intake manifold (which doesn't seem very likely).

Or maybe my brain is just too tired to think of a better idea.

Cheers,

Don

Ok, here's my WAG ....You have carbon buildup on the throttle plate / body and when you stomped on it, the butterfly got temporarily jammed up on the carbon residue and stuck open

:icon_duck:

Larry

That's the only thing that makes sense Larry, my car has been fine since.

Might be worth a peek in there Roger?

You can do a OK job of cleaning the butterfly and throat by removing the big bellows and going at it with some some carb cleaner and a rag ...don't want it to happen again, say, in the local parking lot or your driveway ....:icon_redface:

Larry

I had similar happen yesterday afternoon, not as extreme but the throttle stuck open nonetheless. I found two things wrong and both involved the vacuum bellows
on the speed control. The linkage inside the bellows was sticking and what I think is the true problem. The linkage from the bellows was attached incorrectly to the throttle plate.
It was mounted to the top instead of the bottom of the throttle plate attachment. I didn't check this the first time I had a stuck throttle.